Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more filly describe the state of the art to which this invention pertains.
There has been considerable interest in recent years in developing high energy density cathode active materials and alkali metals as anode active materials for high energy primary and secondary batteries. As the evolution of batteries to higher gravimetric and volumetric capacities and smaller sizes continues, there is increased interest in batteries of non-cylindrical shapes, particularly batteries with prismatic shapes or at least two substantially parallel flat sides. When the prismatic cells in these batteries utilize a liquid electrolyte, this electrolyte is commonly added to the prismatic cell after the assembled electrode stack has been placed in an enclosure, such as a flexible bag or a rigid metal casing, and sealed except for a small opening through which the liquid electrolyte is introduced. After the liquid electrolyte has been filled into the bag or metal casing, the small opening of the fill port is then sealed to completely seal the cell, as, for example, described in U.S. Pat. No. 5,439,760 to Howard et al. and U.S. Pat. No. 5,549,717 to Takeuchi et al., and in European Pat. Application No. 851,522 A2 to Inuzuka et al.
This general method of making prismatic cells that utilize liquid electrolytes may be adequate for some battery product designs and electrochemical chemistries such as, for example, some electroactive transition metal oxide cathode active materials that undergo a intercalation with lithium ions during their electrochemistry and are highly porous in combination with liquid-permeable or porous current collector substrates for the cathode which promote the penetration and filling of the cell by the liquid electrolyte. For example, U.S. Pat. No. 5,478,668 to Gozdz et al. describes prismatic cells where one of the current collector foils of the cell has an open grid and is permeable to allow penetration of electrolyte solution into the cell layer. However, penetration of the electrolyte solution into the cell layers may not be effective when the porous areas of the separator and of the cathode active layer are difficult to fill with the liquid electrolyte such as, for example, may occur with very dense cathode active layers or with non-permeable substrates or current collectors for the anode and the cathode. Also, it may not be effective when excess amounts of liquid electrolyte are introduced into the cell and add undesired weight and further produce undesired loss of soluble cathode active materials from the cathode active layer such as, for example, are formed in the oxidation-reduction electrochemistry of many sulfur-containing cathode active materials. Further, it may not be effective as the anode, cathode, separator and other layers of the prismatic cell become thinner and produce more layers and a greater surface area in each prismatic cell that requires filling with the electrolyte.
In the preparation of prismatic cells utilizing the general process of preparing the electrode stack in a prismatic shape, enclosing the electrode stack in a barrier material film and sealing except for a small opening through which to fill the sealed bag with liquid electrolyte, filling with the electrolyte, and then completing the sealing of the bag, it is known to wind the anode, separator, and cathode layers of the electrode stack of the cell on a mandrel of some type and then press the electrode stack into a prismatic shape. Subsequently, the pressed electrode stack is sealed in the bag or rigid casing material and then the liquid electrolyte is introduced into the sealed bag or rigid casing. A variety of shapes, including flat, rounded, and rhombic shapes, have been utilized for the mandrel to wind the electrode stack before pressing into the prismatic shape. For example, U.S. Pat. No. 5,439,760 to Howard et al. discloses the use of elongated, very thin, and flat mandrels for winding into prismatic cells. U.S. Pat. No. 5,603,737 to Marincic et al. describes conventional methods of preparing rectangular-shaped or prismatic cells, such as stacks of individual electrode layers, a zig-zag configuration of electrode layers, and a folded configuration of electrode layers. The '737 patent discloses a thin flat mandrel for winding electrode layers for mounting within a rectangular-shaped housing of a cell. U.S. Pat. No. 5,549,717 to Takeuchi et al. describes the use of a flat mandrel of rectangular cross-section to prepare prismatic cells. Also, for example, U.S. Pat. No. 5,658,683 to Kageyama et al. describes winding around a core which has a rhombic sectional shape, compressing to form a multilayered roll having an ellipsoidal sectional shape, placing the roll in a casing, and filling the casing with a non-aqueous electrolyte. The '683 patent is directed to applying the multilayer roll to a rectangular box-shaped or prismatic-shaped cell and states that the general method of winding the multilayer roll around a core having a circular or ellipsoid shape in section produces undesirable winding looseness between the electrode layers which leads to reduced and less uniform discharge capacity in the cell. The diameter or circumference of these mandrels relative to the desired dimensions of the prismatic cell have not been specified. As prismatic cells utilize thinner cathode, separator, and anode layers and consequently contain more total layers and a greater surface area of layers, often with more fragile, thinner layers that need protection from mechanical stresses during fabrication of the cells, suitable mandrel shapes and circumferences need to be established for effectively obtaining consistent results in the preparation of prismatic cells.
Despite the various approaches proposed for the methods of preparing high energy density prismatic cells, there remains a need for improved methods, which provide a combination of excellent filling by the electrolyte and high electrochemical utilization of the cathode active material together with consistent product performance while reducing the amount of excess electrolyte introduced into the prismatic cell and also reducing the mechanical stresses that may damage the multilayers of the prismatic cell.